Posted
by
kdawson
on Sunday November 19, 2006 @12:24AM
from the deep-armageddon dept.

aluminumangel writes, "Taking a page out of a Michael Bay movie, NASA is considering a manned mission to land on an asteroid, 'poke one with a stick,' and see how feasible it would be to deflect it from its course. Obviously, the application would be valuable in a doomsday situation and hopefully could keep us from going wherever the dinosaurs went." The article makes oblique reference to another goal such a mission could serve: giving us something to do in space, something to engage the paying public, between the time we return to the Moon and the time we get to Mars.

If we can good at altering asteroid's paths, we could use near earth asteroids as ramming tools. We should ram a few into the same spot on Mars and get a nice deep crater. We get practice diverting asteroids and learn more about deeper martain soil.

I'm a fan of space and staying busy till the end times come, don't get me wrong, but what can poking a comet tell us that we wouldn't be able to figure out using the known laws of physics and, you know, science and stuff....

As an exercise for my high-school physics students studying energy and momentum conservation, I had them run the numbers on the scenario from the movie "Armageddon" for an asteroid "the size of Texas", taking this to mean in separate cases the area of Texas with a range of densities, etc.

Giving the astronauts every benefit of the doubt (able to intercept it twice as far out as they did in the movie, bomb able to be placed at the center of mass, the bomb having ten times the yield of largest nuke ever exploded by man, perfectly elastic explosion, etc. etc. etc.) they not only couldn't make the asteroid miss the Earth, they would only have changed impact points by about a meter!

I love sci-fi movies and like to give my students problems from popular films that illustrate the absurdity of Hollywood stories.

The proposals are at an early stage, and a spacecraft needed just to send an astronaut that far into space exists only on the drawing board

Actually the apollo stack (SM, CM, LM ascent and descent stages) had easily enough velocity budget to fly to and return from some near Earth asteroids. It didn't have the consumables to do it but that could have been launched separately. You get more redundancy that way.

Of course we don't have the apollo CM, which is the only spacecraft in existance which could make a high speed return from an asteroid and reenter the atmosphere, but we will have the CRV which should have similar capabilities. The saturn 5 launch system doesn't exist either and thats the part of this system which is really vapourware.

Anyway good luck to them. Mars has been held off for so long because it is so much more risky and difficult than the moon. Asteroids offer progressively harder challenges, minus the risk of sudden death landing a heavy vehicle on mars.

That would be impossible to time correctly, though, as you'd have no control over the velocity and possible fragmentation, and no abort option when your calculations are off by a fraction. And the target (presumably a nation) might see it coming more than a mile away and have time to retaliate or shoot the rest of us just for fun - I would, anyway.

If you want to destroy a city, just carpet bomb it. Blowing up cities is easy. The point is that any nation that has the ability to move an asteroid (read that as the US, the US, and the US) already has the ability to wipe out cities at will. At the stupidly insane cost of moving an asteroid, you might as well just build a few thousand cruise missiles and level the city that way. The only use moving asteroids has is for mining purposes and throwing at planets in an effort to drop some water on it (and even then, you probably want to use a comet).

There was an article on Slashdot a while back on a new crater discovered in Antarctica. It was a couple hundred miles across and was believed associated in some way to the Great Extinction. Well, there's a neat website that lets you calculate the size of crater and damage done for a given size of asteroid. It took a while to find one that would produce the crater observed that would have a combination of speed and size that would leave anything left alive at all, let alone 5%-10% of the biomass.

My crude reverse-engineering of the asteroid suggests that it would have to have been moving very very slowly compared to the Earth, and be about 50 miles across. Even so, the calculator predicted that anything within the horizon of such an impact would be instantly vaporized and that the entire hemisphere would be subject to earth tremors of magnitude 11.2 or above. That was about the smallest-scale devastation I could find that would produce the right-sized crater.

(Faster asteroids would be smaller, for the same-sized crater, but end up releasing much more energy, as energy goes up with the square of the velocity.)

Now, turning an asteroid (or comet) is plausible, but it has to be done early. You say you can only achieve a meter or so, but in reality that doesn't mean anything. You change the trajectory, and the change of displacement is then the distance the asteroid travels divided by the tangent of the angle between the original path and the new path. (The tangent is equal to the opposite over the adjacent - SOH CAH TOA. You make the adjacent the line it would originally have followed and the opposite becomes the displacement.) Objects travelling along a curved trajectory need to be mapped into a linear system first, which is usually a very simple transform.

So how does this help? Well, since you are changing an angle, the implication is that if you increase the distance away you make this change, you will increase the displacement from the original position. If the change in displacement exceeds the Earth's radius plus the safety margin needed to prevent the Earth's gravity from causing the collision to occur anyway, then it makes bugger all difference if you can make one degree of change or one billionth of a second of a degree. All that matters is that the cumulative change places the body outside the danger zone.

What does this mean in practice? In practice, it means that if it's just about to collide, there is nothing you can do to stop it and there are few structures in the world capable of withstanding 11.2 magnitude tremors. Evacuating the hemisphere and placing everyone on a geologically-sound plateau would be far cheaper and would have a much better chance of success. Near-zero, as opposed to absolutely zero.

If the body is unlikely to collide for a couple of orbits and a few hundred years, then you can talk about serious landscaping the solar system. That's the kind of distance where even a small angle will make a large difference. Better yet, gravity is vastly more powerful than any explosion - if you can shift the orbit just enough to place the body close to a large planet, the total deflection will vastly exceed anything explosives can achieve. Gravity is a significant force on these scales.

This all assumes that the body is solid, of course. The Japanese robot probe that landed on an asteroid not too long ago found a nearby asteroid whose density was unimaginably low - it is likely to be nothing more than space grit held together with collective gravitational attraction where the packing is no better - and probably worse - than coarse-grain sand. It could be said that its structure is best described as sheer damn luck. You fire off a nuke on something like that and there's no telling what will happen, other than most of the energy will go straight through it. At this point, we simply don't have anything like a large enough catalog of asteroids, nor in anything like sufficient detail, to know if this is a freak accident or the norm. Until you know enough of the basics, you can't know anything about the complexities.

I'm surprised that no one has mentioned the obvious: Using an asteroid landing as a precusror to a mining mission.

If NASA's plans go forward, they're going to need a space infrastructure. Eventually, that will mean space-based manufacturing. For manufacturing, you need raw materials. Those raw materials are expensive to lift from Earth's gravity well. Ergo, the best solution is to mine them from much smaller gravity wells where the cost of transport is comparitively minimal.

The key issue that an mission to an asteroid would need to resolve is the actual composition and concentration of valuable ores. Scientists currently have a lot of educated guesses, but we won't know for sure until a geologist makes a proper survey.

Launching ANYTHING into space space is stupidly expensive. Launching something into fast enough to escape earths gravity is hard and very expensive fuel wise. Launching a payload is even harder and more expensive. Further, the 'pain bomb' method is hardly an exact science and sure as shit will not have city precision. Even if someone really felt like blowing the money it takes to launch something like that into space, it sure as hell would not be a stealthy maneuver. The nuclear armed nations of the world (with the exception of the new members to the club) have systems for detecting launches. Everyone would know if someone fired something out of Earth's orbit.

Seriously, if you want to wipe out a city at 1/100 of the cost and half the time, just do it the old fashion way and bomb it into dust.

The difference with having thousands of tiny asteroids is that due to the incresed surface area, they would burn up in the atmosphere.

I don't agree. If you have enough tiny asteroids, you are going to heat up the atmosphere, which lowers its density, which makes it less efficient at stopping all the other little asteroids. The first ones will burn up. The last ones will hit the ground. And you'll have a lot of superheated air to deal with. The amount of energy remains the same, and earth is going to have to absorb it. You canna change the laws of physics Jim!